They’ve created a wiki to support those trying to make mods, and this page has a breakdown of what the tools can and can’t do. Can’t do: change basic core gameplay; add new skills or attributes “at the moment”. Can do: everything else. The list of changeable elements includes tweaking existing skills, plus adding conversations, artwork, characters, levels, AI behaviour, weapons, enemies… seemingly near-everything.

If you’re just looking to play other people’s creations, there’s Steam Workshop support for those who own the game through the service. There’s nothing available to play yet, so if you can’t wait and need an excuse to re-play the game, the update also comes with an extra “++” hard mode.